Although not limited to such applications, the invention will be more particularly described with reference to frames used for keeping a space between two glass sheets in the production of plane and hermetically sealed glass envelopes, such as field-emission screens, and therefore for keeping a space of constant, but not limited, thickness of generally less than a few millimeters around the periphery of the glass sheets.
Such a configuration is widely used for the production of display screens. These may be field-emission display (FED) screens having cathodes consisting of carbon microspikes or nanotubes or screens of the flat cathode-ray tube (flat CRT) type. Such a configuration may also be desirable for producing vacuum glazing or plane lamps. The expression plane lamps should be understood to mean those encompassing lamps used in the manufacture of portable computers and larger-sized lamps for the production of advertising panels or partitions, for example in offices, whatever the technology used for these lamps.
In a screen of the field-emission type, a limited space, in which a vacuum is created, must be maintained between the two glass sheets. It is therefore necessary around the periphery of the screen to provide means which make it possible to define and maintain this space and which furthermore seal the volume thus formed by the glass sheets and these peripheral means.
It is known, for example for the production of vacuum double glazing as described in Patent Application EP-A-0 645 516, to provide an enamel-type sealing paste deposited to the desired thickness around the periphery of the two glass sheets in order to join them together, sealing the space defined between the two glass sheets; the thickness and the hardness of the enamel obtained after firing the latter enable it to help to keep the desired thickness of the vacuum layer produced between the two glass sheets. Although such a construction is possible if the desired height of the space created between the two glass sheets does not exceed 0.5 mm, above this value it is no longer possible to guarantee a perfect seal, especially in the case of industrial production. And, in the production of field-emission screens, the said height is at least 0.7 mm.
A technique currently used for producing the peripheral element, starting from a glass sheet having the desired thickness for the said element, consists in producing a frame whose edges have a width of the order of one centimeter. To do this, the central part of the glass sheet is cut out and removed. It is then possible to join the glass sheet thus obtained to the two glass sheets constituting the faces of the screen by means of an enamel paste or sealing frit which fulfils the sealing function. Although this construction is satisfactory from a technical standpoint, it has various drawbacks in its implementation. Firstly, such glass frames must be produced before the screen is manufactured, and this requires handling of the screens, which is not simple because they are fragile. To remedy this and at the very least to reduce the risk of breakage, the width of the edges is made large enough to increase the stiffness of the frame. However, this stiffening is achieved to the detriment of the final viewing area for a given screen size, since the frame will encroach on this viewing area. Furthermore, such frames are relatively costly to obtain because of the loss of material occasioned by the central recess of the glass sheet from which these frames are cut. Finally, this technique is limited to the production of frames having a rectangular cross-section, in particular preventing the frame from being given other functionalities useful for improving the performance of the final product.
The invention thus relates to means making it possible to define and maintain a space between two plane substrates, which help to seal the volume bounded by the substrates, and the means being at least equivalent in terms of technical results to the latter solution presented above and which advantageously do not have the drawbacks of the abovementioned solutions. Advantageously, they should be able to be produced in all thicknesses corresponding to the applications described above and for costs lower than those mentioned above. Furthermore, the dimensions of these means should be smaller than the latter technique mentioned above so as to optimize the viewing area of the end product, for example a screen.
The invention also relates to a glass frame whose cross-section has a substantially polygonal profile that includes at least one re-entrant angle.